Method of stabilizing noncrystalline piezo-electric microphone diaphragms



' Nov. 1, 1932.

A. MEISSNER 1,886,235

METHOD OF STABILIZING NONCRYSTALLINE PIEZO ELECTRIC IICROPHONE DIAPHRAGMS Filed Sept. 10, 1929 j'q'gli m P R Maul-W7 3/ 6/ 7/46 INVENTOR KLEXANDER MEI sum BY 4 yam/L ATTORNEY Patented Nov. 1, 1932 UNITED STATES PATENT OFFICE ALEXANDER HEISBNEB, 0] BERLIN, GERHA NY, ASSIGNOB TO TELEIUNKEN enam- BCHAI'T FUR DRAH'ILOSE TELEGRAIPHIE TION OI GERMANY I. B. H 0] BERLIN, (3mm, A CORPORA- KETEOD 0] STABILIZING NONCBYBTALLINE PIEZO-ELEO'I'BIG IIOBOPHONI DIAPHBAGMS Application filed September 10, 1829, Serial No. 891,585, and in Germany August 1, 1928.

This invention relates to the piezo-electric art and deals more specifically with the production of piezo-electric materials from noncrystalline substances.

In accordance with the principles of thls invention it has been found experimentally that if such materials as resins, waxes, as haltum, or the like, are heated and then all bwed to cool, direct current potential being applied to the body during coohng, the resulting material exhibits good piezo-electric properties and may be utilized 1n high grade microphones or in other cases where the present day crystalline piezo-electric materials are used.

It has also been found, in accordance with the principles of this invention, that the piezo-electric properties of the body thus produced may be substantially increased by the addition of a powdered piezo-electric body such as quartz, for example. 7

The manner in which the present invention is carried out will be more clearly understood from the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a diagrammatic view showing a system embodying the principles of the present invention; and

Fig. 2 is a diagrammatic showing of the properties of piezo-electric elements in accordance with the principles of this invention.

Referring in detail to Fig. 1, a piezo-electrio plate or diaphragm P is connected to the input circuit of a vacuum tube amplifier R. It will be understood that the present figure discloses, by way of example, the use of the piezo-electric element as a microphone. Obviously, the piezo-electric body may be used, in accordance with the principles of the present disclosure, for all purposes for which piezo-electric bodies are now used, such as, for example, as a means to maintain constant the frequency of an oscillation generator, a frequency standard, or other uses.

It has been found, in accordance with the principles of this invention, that a piezoelectric body of the type herein disclosed loses its piezo-electric action or moment in the course of time but that such decrease in piezoelectric action can be either entirely eliminate or retarded to a substantial degree by applying across the terminals of the piezoelectric ody a direct current potential. In the figure the direct current source B is shown permanently connected to the terminals of the crystal P. In order not to im air the operation of the vacuum tube R a locking condenser C is applied between the the vacuum tube and one terminal of the crystal, thus cutting off the direct current potential between tne filament and grid of the vacuum tube R. i

The condenser C is proportioned so that the alternating current potentials, created when the plate or diaphragm P responds to voice waves, are allowed to act upon the grid grid of of the tube R in a manner well understood.

It is important, in accordance with this invention, t at the battery potential B be applied to the plate 'P in the proper direction, that is, the polarity of the battery must be properly chosen with respect to the plate. In this connection, it has been discovered that for some materials, for instance, sealing wax, the piezo-electric action will always maintain the identical sign. In otherwords, the piezo-electric action corresponds to the potential which was applied to the plate when the latter was being formed. Hence, in order to preserve and maintain the piezo-electric action the potential furnished from battery B must be of the same sign or sense of flow as was the former potential. Other substances,

such as electret, asphaltum, etc., whether with or without an addition of powdered quartz, exhibit the peculiar property that immediately after forming they exhibit the piezoelectric action or moment in the same sense as the source of forming potential.

After a few days the value of this moment drops to zero. Thereafter, however, it grows again but in the opposite direction and the permanent condition finally is that the plate exhibits a moment having the opposite sign as compared with that which it had immediately after the discontinuance of the forming process. Fig. 2 shows graphically the variation of the piezo-electric moment for substances of this kind plotted against the time.

In the technical press it has become customary to desi ate a substance of aspecial nature in whic the permanent electric moment is reserved through years, as electret. This su stance has been more exhaustively investigated by M. Eguchi, Phil.- Mag, 4, 1925, p. 178, and it consists of a mlxture of 45% colophony, 45% carnauba wax and 10% beeswax. This special material having a composition as just indicated is meant in the present application as electret. The property of the latter to carry permanent electr c momentums, as can be seen from the application, is possessed by a number of other substances, for instance, different kinds of resins,

, waxes, sealing wax and asphaltum.

It has been found, also, that this final state, after the reversal of polarity, in some materials decreases gradually but that the positive maximum of the moment is slightly greater than the moment in the negative direction. It therefore becomes desirable to provide an arrangement which is adapted to maintain this positive maximum of the piezoelectric action. To accomplish this result it has been found advisable to apply a direct current potential after the formmg process of a plate though in the opposite sense as compared to the potential applied before.

' It will be understood that various changes in the specific embodiments disclosed herein will readily suggest themselves to those skilled in the art and the present invention, therefore is not to be limited except as dethe appended claims.

Having thus described my invention, what 1. A iezo-electric element comprising a ized in that the potential 'is-appliedfi'n'imev diately after forming the plate, and that the sense of said potential is'the same as that of the forming potential.

8. Method according ized in that the potential is only applied to the plate after a reversal in polarity has taken place in the plate subsequent to forming, and that the sense of said otential is opposite that of the potential use during the forming process.

9. In combination, an electron discharge device having a cathode, anode and control electrode, a non-crystalline piezo-electric body connected to said cathode and control electrode, a source of direct current potential located across the plates of said piezo-electric bod and a blocking condenser located interme iate said control electrode and said body. r

ALEXANDER MEISSNER.

5. In the method of preparing a piezoelectric body, the steps of heating the material from which said body is to be formed, permitting said body to cool and maintaining a-direct current potential upon said body du'ing the coolin operation.

ethod o stabilizing a non-crystalline body possessing piezo-electric properties WhlCh comprises applying a unidirectional potential to the electrodes of said body.

7. Method according to claim 6, characterto claim 6, character- 

